The present invention relates to the technical field of aerogels, especially silica aerogels.
In particular, the present invention relates to a method for producing aerogels and also to the aerogels obtainable by the method of the present invention and their uses, particularly in insulants. The present invention further relates to a method for drying aerogels.
Silicate-based aerogels, i.e., silica aerogels, derive from orthosilicic acid H4SiO4 and its products of condensation. They are highly porous solids having a pore volume of generally 95% to 99.8% by volume, based on the total volume of the aerogel. Owing to their high porosity, aerogels are poor conductors of heat and sound and as such of interest for the development of insulating and isolating materials.
However, aerogels are largely confined to some specialty applications in the area of isolant and insulant materials, since their high porosity makes aerogels extremely fragile solid-state structures destroyed even by relatively low mechanical stress. In addition, the preparation of aerogels is very costly and inconvenient, explaining why aerogels have hitherto not been commercially useful in isolating and insulating materials.
Initial syntheses of silica aerogel are known to date from the 1930s. Silica aerogel is typically produced by the sol-gel process. The starting point for a conventional synthesis of aerogel is a dilute sodium silicate solution, which is acidified with hydrochloric acid to precipitate an amorphous gel known as a hydrogel. Said gel is dried in an autoclave under supercritical conditions, which makes this process very inconvenient and costly, to obtain an aerogel. Supercritical drying of the aerogel, or removal of the solvent, is to necessary because otherwise, owing to the highly porous structure of the aerogel, the capillary forces acting during solvent removal destroy the solid-state structure of the aerogel.
Alternative methods for producing silicate-based aerogels proceed from the hydrolysis of organosilanes, for example tetramethoxyorthosilicate and/or tetraethoxyorthosilicate, and likewise involve a step of forming a hydrogel or, to be more precise, an alkogel and a step of removing the solvent in the supercritical domain. In developments seeking to simplify the process and improve its energy efficiency, the solvent or solvent mixture can be replaced by carbon dioxide which in turn is removed by supercritical drying. But even this version of the process is too inconvenient for cost-effective practice on an industrial scale.
There have additionally also been proposals seeking to develop approaches based on drying the aerogel under more moderate conditions. They generally involve a step in which the hydrogel/alkogel obtained is subjected to hydrophobicization, especially by silylation, for example with dimethylchlorosilane or trimethylchlorosilane. This is followed by a solvent exchange of the polar solvent present in the reaction mixture for an apolar solvent in order to yet further reduce the surface tension and thus the capillary forces. Such surface-modified hydrogels/alkogels are convertible into aerogels by distillatively removing the solvent and subsequently drying the hydrogel/alkogel at temperatures above 100° C. While the capillary forces cause the aerogel to shrink during the drying process, it is not destroyed and returns to its original shape on completion of drying.
Since, however, these methods are also very inconvenient and time intensive and, what is more, they generally lead to aerogels barely capable of withstanding mechanical stresses, there are endeavors seeking to further optimize the production methods for aerogels and also the physical parameters of aerogels.
DE 195 38 33 A1 thus describes a method for subcritical production of aerogels wherein hydrosol is sprayed into paraffin to create sol spheres having a predetermined diameter and, after gel formation, the spheres are again treated with a polysilicic acid solution. This is optionally followed by a hydrophobicization and a subsequent solvent exchange. Finally, the aerogel obtained is dried under supercritical conditions.
DE 195 41 992 A1 relates to a method for producing inorganically modified aerogels by use of alcohols, which comprises adding an inorganic acid to an aqueous waterglass solution to produce a hydrosol and removing the resultant salts to a very substantial extent. The gel is subsequently washed with an organic solvent in order to get the water content below 5 wt %, which is followed by a surface-modifying step and a subsequent step of drying the gel obtained.
DE 196 48 798 C2 further relates to a method for producing organically modified aerogels by surface-modifying the aqueous gel without prior solvent exchange and then drying.
DE 197 52 456 A1 relates to a method for producing organically modified aerogels starting from silicon tetrachloride.
Lastly, EP 0 171 722 A1 relates to a method for drying hydrogels, specifically silicate-based hydrogels, wherein the first step comprises exchanging the water content of the hydrogel for methanol, which is then in turn replaced by carbon dioxide. The carbon dioxide is finally removed under supercritical conditions.
All these methods, however, fail to provide a particulate, mechanically sufficiently stable aerogel under economically sensible conditions which is potentially capable of achieving wide use in isolating and insulating materials.
Moreover, the prior art methods reviewed are all without exception complex multi-step processes which are typically possible only with the use of numerous organic solvents and further additives. Since the chemistries used are often corrosive, poisonous and/or flammable, or used under high pressure, special safety precautions have to be taken to handle these chemistries and to dispose of them. This drives up the costs of aerogel production still further, resulting in silica aerogels currently having to be priced at up to € 120/kg.